1. Field of the Invention
This invention relates to an alternator for a vehicle such as a passenger automotive vehicle or a truck.
2. Description of the Related Art
To reduce the aerodynamic resistance while traveling, a vehicle body tends to have a slanted nose shape. Securing a sufficient residential space for a passenger compartment is earnestly demanded. To satisfy these requirements, engine rooms of automotive vehicles have recently been becoming so narrow and crowded that only a limited space is available for installing an alternator. Meanwhile, to improve fuel economy, the rotational engine speed tends to be reduced during an idling condition. The rotational speed of the alternator decreases in accordance with the reduction of the rotational engine speed. On the other hand, there is a need for meeting the increasing electric loads caused by safety control devices or others. Thus, an alternator which generates a lot of power is strongly required. In other words, a compact high-power alternator for a vehicle is required. Also, an inexpensive alternator for a vehicle is desired.
It is also socially desirably to reduce the noise leaking from automotive vehicles into exteriors thereof. In general, the passenger compartments of automotive vehicles have been made quieter to increase the values of the vehicles. Accordingly, vehicular engine noise has been reduced. Magnetic noise generated by a vehicular alternator is more easily sensed as vehicular engine noise is reduced. Thus, a low-magnetic-noise inexpensive alternator for a vehicle is desired. Also, a compact high-power alternator for a vehicle is required.
In a general alternator (a prior-art alternator) for a vehicle, as shown in FIG. 12, a rotor contains a Lundel-type iron core (referred to as the pole core hereinafter) having a cylindrical portion, a yoke portion, and a claw-like magnetic pole portion. The entire length of the general alternator is determined by the axial-direction length (referred to as the axial length hereinafter) of the rotor. Accordingly, a reduction in the axial length of the rotor is desired for a compact alternator design.
In the rotor of the general alternator, as shown in FIG. 12, magnetic flux .PHI. flows from the cylindrical portion to the yoke portion and the claw-like magnetic pole portion, gradually advancing from the claw-like magnetic pole portion to a stator iron core. The magnetic flux .PHI. generated from the rotor is given as follows. EQU .PHI.=Mf/G
where "Mf" denotes a magnetomotive force, and "G" denotes the sum of the magnetic resistances of respective portions. The magnetomotive force Mf is equal to the product of a current flowing in a field coil and the number of turns of the field coil. The magnetomotive force Mf is proportional to the product of the cross-sectional area of the field coil and the temperature of the field coil. Each of the magnetic resistances is proportional to the length of a magnetic path which is divided by the cross-sectional area of the magnetic path.
In the prior-art structure of FIG. 12, the magnetic-path cross-sectional areas S1, S2, and S3 at different portions of the pole core are set substantially equal to each other to prevent the occurrence of local magnetic saturation. The dimensions of the portions of the pole core are chosen to provide a proper space for the field coil which can generate a desired magnetomotive force. The cross-sectional area of a magnetic path in the stator iron core is made substantially uniform in correspondence with the magnetic flux generated by the rotor. The cross-sectional area of each slot in the stator iron core is decided on the basis of the resistance of a winding. As a result, the axial length of the stator is also determined.
In a prior-art magnetic circuit which is designed in such a way, the axial length L3 of the cylindrical portion of the pole core is substantially or approximately equal to the axial length L1 of the stator iron core as shown in FIG. 12.
In the prior-art structure of FIG. 12, when an increased alternator power output is required, the magnetic flux .PHI. generated by the rotor is increased. To implement the generation of increased magnetic flux, it is necessary to increase the magnetomotive force Mf or to reduce the magnetic resistances.
In the prior-art structure of FIG. 12, to increase the magnetomotive force Mf, it is necessary to increase the cross-sectional area occupied by the field coil or to enhance the cooling performance of the field coil. If the cross-sectional area occupied by the field coil is increased without changing the size of the alternator, the cross-sectional areas of other magnetic paths need to be uniformly reduced. The reductions in the cross-sectional areas of the magnetic paths result in increases in the magnetic resistances. The increased magnetic resistances cause a reduction in the generated magnetic flux .PHI.. If a greater cross-sectional area of the magnetic path is required to reduce the magnetic resistance, it is necessary to reduce the cross-sectional area occupied by the field coil. Thus, the prior-art structure of FIG. 12 needs to be designed in consideration for a trade-off between the two requirements.
Japanese published unexamined utility-model application 5-11769 (corresponding to U.S. Pat. No. 5,233,255) discloses a general structure having built-in-type cooling fans which are fixed to two magnetic pole side surfaces of a rotor for cooling a field coil. In the general structure of Japanese application 5-11769, the rotor has an approximately flat shape such that at a rotor cross-section, the width of two side surfaces of the pole core is smaller than the radial-direction height. Thus, the area of contact between the field coil and the pole core is increased to enhance the thermal conductivity. As a matter of fact, it is difficult to improve the cooling performance for the following reason. Bridge portions (referred to as the coil ends hereinafter) of an armature coil which are located at two axial-direction side surfaces of a stator iron core are opposed and adjacent to base portions of magnetic pole claws of a pole core in a radially inner side thereof. Since an alternating current flows through the armature coil, alternating magnetic flux occurs therearound. The magnetic flux flows into the base portions of the magnetic pole claws of the pole core. The pole core is made of iron. Thus, an eddy current occurs therein, and heating takes place. Among parts of the alternator, the armature coil is a heating source having the highest temperature. Heat is radiated from the coil ends of the armature coil to the base portions of the magnetic pole claws of the pole core. Therefore, the cooling performance of the pole core is decreased, and the conduction and the radiation of heat from the field coil to the pole core are impaired. Thus, it is difficult to lower the temperature of the field coil.
In a prior-art designing method, if the magnetic-path cross-sectional area of one portion of a rotor or a stator is increased while the magnetic-path cross-sectional areas of other portions remain unchanged, the magnetic flux .PHI. generated by the rotor is hardly increased since magnetic saturation of a portion with a high magnetic flux density restricts the whole magnetic flux. Thus, an alternator power output per alternator weight is hardly improved. Enlarging the magnetic-path cross-sectional areas of all portions is contrary to the miniaturization of the alternator.
The claw-like magnetic pole portions of the pole core have a natural frequency with respect to swinging vibration in radial directions. When a magnetic force acting between the stator iron core and the rotor pole core is tuned to the natural frequency, sound from the claws (i.e., claw noise) occurs.
In a general prior-art alternator having a 12-pole rotor core and a stator iron core with 36-magnetic-pole teeth, as shown in FIG. 13, claw sound with a high level occurs at an alternator rotational speed of 11,000 rpm. Such high-level claw noise is annoying.